This invention relates to ink jet printing and more particularly to an ink jet printer having improved ink droplet placement during printing of an image by the printer's droplet-ejecting printhead on a recording medium which is electrostatically tacked to a dielectric transport belt.
Conventional ink jet printing systems use various different methods to produce ink droplets directed toward a recording medium. Well known devices for ink jet printing include thermal, piezoelectric, and acoustic ink jet printheads. All of these technologies produce roughly spherical ink droplets having a 15-100 .mu.m diameter directed toward a recording medium at approximately 4 m/sec. The ejecting transducers or actuators in the printheads, which produce the ink droplets, are controlled by a printer microcomputer or controller. The printer controller activates the transducers or actuators in conjunction with movement of the recording medium relative to the printhead. By controlling the activation of the transducers or actuators and the recording medium movement, the printer controller directs the ink droplets to impact the recording medium in a specific pattern, thus forming an image on the recording medium.
All droplet-on-demand type printheads produce ink droplets theoretically directed toward the recording medium in a direction perpendicular thereto. In practice, however, some ink droplets are not directed exactly perpendicular to the recording medium. The ink droplets which deviate from the desired trajectory are undesirable since such misdirected droplets impact the recording medium at a location not anticipated by the printer controller. Therefore, misdirected droplets affect the quality of the printed image by impacting the recording medium in unwanted positions.
U.S. Pat. No. 4,386,358 and U.S. Pat. No. 4,379,301 disclose a method for electrostatically deflecting electrically charged ink droplets ejected from an ink jet printhead. Charges placed on electrodes on the printhead, as disclosed herein, are controlled to steer the charged ink droplets in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink droplets, the method disclosed in these patents compensates for ink droplet misdirection caused by the known printhead movement when the ink droplet is ejected. However, the electrostatic deflection method disclosed in these patents does not compensate for unanticipated or unpredictable factors, which can affect ink droplet trajectories.
U.S. patent application Ser. No. 08/480,977 filed Jun. 7, 1995 entitled "Electric-Field Manipulation of Ejected Ink Drops in Printing" and assigned to the same assignee as the present invention, discloses electrodes behind the recording medium and/or on the printhead face to induce charges on the ejected ink droplets and accelerate them toward the recording medium. By appropriately controlling the electrostatic deflection of the ink droplets created by each column of actuators in the printhead, the droplets are selectively directed to impact the recording medium at positions both left and right of a center position, so that each actuator can create up to three vertical print columns of spots on the recording medium, thus enhancing the printing resolution of the device.
U.S. patent application Ser. No. 08/721,290 filed Sep. 26, 1996 entitled "Method and Apparatus for Moving Ink Drops Using an Electric Field" and assigned to the same assignee as the present invention, and discloses a method to reduce the input energy required for droplet ejection and thus reduce printhead operating temperature. This is achieved by using an optimal electric field to accelerate the droplets subsequent to ejection. An important advantage is the reduction in power consumed, and the resulting prolonged useful lifetime of the printhead. The optimal electric field also accelerates both main drops and satellite droplets toward the print medium, so that they arrive at the same time.